1. Field of the Invention
The present invention relates to a current-carrying control device which controls a switching condition (conductible condition or cutoff condition) of a current-carrying line by a relay such as electromagnetic relay or semiconductor relay, or to an electric power steering apparatus including such the current-carrying control device. More particularly, the present invention relates to a device capable of preventing an overheat of the relay practically.
2. Description of the Related Art
In the electric power steering apparatus used for a small vehicle such as light car, a steering torque generated at a steering shaft by steering a handle of the vehicle is sensed by a torque sensor. Corresponding on the steering torque, a steering-assistance torque is generated by flowing a current to an auxiliary motor (hereafter simply called motor) attached at the steering shaft from a battery of the vehicle. In current control of the motor for the torque generation, an H-bridge circuit including usually four FETs (Field Effect Transistor) is used, the motor is driven by a control portion including a microcomputer with PWM (Pulse Width Modulation) system through a driving circuit including the H-bridge circuit.
In the current-carrying line of the driving circuit (if circumstances require, current-carrying line between the driving circuit and a power source, and current-carrying line between the driving circuit and the motor), a relay such as electromagnetic relay is provided as similarly as a general motor control device so as to control the switching condition of the current-carrying line by the control portion. That is, in the case of the electronic power steering apparatus, at a non-operation condition of the apparatus, the current-carrying line is made the cutoff condition by keeping the relay an open state so as to avoid generation of a large current (over-current) by a reverse connection of the battery (to connect the battery of the vehicle with a reverse polarity). In case of a short fault in the FET at an operation of the apparatus, the relay is switched to the open state so as to cut the current-carrying line in order to avoid generation of the large current, malfunction of the motor, or regenerative lock caused by the fault.
The regenerative lock means here phenomena that both terminals of a motor coil are connected by the short fault of the FET, and the handle of the vehicle connected to the motor becomes hard or impossible in rotary operation because so called regenerative break at the motor is generated. In order to solve the problem of such regenerative lock, a relay such as electromagnetic relay is necessary to provide even at the current-carrying line between the driving circuit (exactly, the H-bridge circuit) and the motor.
In such electric power steering apparatus, although measures have carried out to protect the FET from heating by controlling the carrying current to the FET (that is, carrying current to the motor.) so that a temperature of the FET is not more than a designated temperature in related art, measures have not carried out to positively protect the relay from heating. This is because of the following. In related art, such electric power steering apparatus has used only for a small vehicle such as light car as above-mentioned because it is difficult to generate a large steering-assistance torque comparing with a hydrolic device. The current flowing through the motor is low level, maximum about 40 A, at a full operating condition of the apparatus such as operation of the quick turn of the wheel. Because of that, a calorific value of the relay or parts generating heat arranged at periphery thereof (the above FET, for example) is not so large, and the overheat of the relay is prevented practically without problem by means of measures that a suitable radiator is provided.
However the above electric power steering apparatus is easy in electric control. There are various merits that a hydraulic pump and an oil piping are not need and the structure is simple. Accordingly, an application for a normal passenger car is investigated nowadays, and it is possible to apply for a large vehicle in future.
When the apparatus is mounted in such comparatively large vehicle, the current of the motor is needed maximum 60 to 80 A for example in order to drive the motor and then generate the designated steering-assistance torque in the full operating condition of the apparatus such as quick return of the wheel. At such condition, the calorific value (in proportion to a square of the current) from the relay itself or parts generating heat at periphery thereof increases substantially, and it is impossible to protect the relay from the overheat with only the operation of the radiator.
A design so that the relay does not become an overheat state under the condition of the above-mentioned current-increase is possible in principle by providing a radiator corresponding to a maximum calorific value at the control unit of the apparatus where the driving circuit, the microcomputer, and the relay are stored or by selecting a capacity of the relay itself for the maximum current. However practically it is impossible. In this case, the radiator becomes large extremely, and the relay becomes large particularly. Because of that, the ontrol unit is not stored at the narrow limit of space for arranging the unit of this kind in the vehicle.
When the relay becomes the overheat condition for example, there is the possibility that the relay coil thereof deteriorates (decrease of life), burns, or a solder connecting the terminal thereof melts. Such overheat condition must be avoided positively and certainly at a viewpoint to make sure of a high reliability of the steering system in the vehicle.
Accordingly, an object of the invention is to provide a current-carrying control device which controls a switching condition of a current-carrying line by a relay such as an electromagnetic relay or a semiconductor relay and an electric power steering apparatus including such current-carrying control device, more particularly, a device for avoiding an overheat of the relay positively and certainly with a small construction.
A current-carrying control device according to the invention has a relay provided at a current-carrying line of a load and controlling the current-carrying line to a conductible condition or cutoff condition by a switching operation of the relay. The current-carrying control device also has a temperature sensor detecting a temperature at a designated portion where a temperature of the relay can be detected or calculated, and a current control unit carrying out a current correction control for correcting down a current flowing through there lay according to necessity based on a detection temperature detected by the temperature sensor in order to keep the temperature of the relay less than an allowable temperature.
Here in xe2x80x9cloadxe2x80x9d, various kinds of electric loads are included such as a motor, a solenoid, and a lamp. xe2x80x9cCurrent-carrying line of loadxe2x80x9d means a series of current path (comprising circuit conductor or cable) between the load and a power source (positive pole and negative pole). In case of that there is a driving circuit as a load, a current path between the driving circuit and the power source and a current path between the driving circuit and the load are included.
In xe2x80x9crelayxe2x80x9d, a usual electromagnetic rely, a semiconductor relay, and a switching element such as FET are included. xe2x80x9cTemperature of relayxe2x80x9d is a temperature of place where it is judged whether by the overheat, a function or a reliability of the relay deteriorates, or is lost or not (that is, whether less than the allowable temperature or not). According to circumstances, is an average temperature of all of relays, or a temperature of a specific place having heat problem of the relay (for example, temperature of the contact of the electromagnetic relay, terminal, or coil).
In xe2x80x9cdesignated portion where the temperature of the relay can be detected or calculatedxe2x80x9d, any portion of the relay itself is included, and a portion being roughly same temperature as temperature of the relay (for example, circuit conductivity connecting terminal of the relay or near part of the terminal at the board) is also included. Further a portion where temperature is changed while a relation with the temperature of the relay is kept (for example, a part comparatively near to the relay at a radiation plate for radiating the heat of the rely) is included. As an extreme mode, a portion equal to circumstance temperature of the apparatus is also included.
xe2x80x9cDesignated portionxe2x80x9d is sure of a portion where the temperature of the relay including a certain allowable error can be detected or calculated. Incidentally in a relay such as an electromagnetic rely usually used for an electric power steering apparatus, as the allowable temperature of a terminal (solder connection part) and coil is a level of, for example, 120xc2x0 C. to 200xc2x0 C., at least error of 1xc2x0 C. to 10xc2x0 C. is practically permitted.
xe2x80x9cCurrent correction controlxe2x80x9d maybe, for example, a control for decreasing the current than usual based on the detection temperature of the temperature sensor (for example, control for surely decreasing the current value only a ratio according to the detection temperature). Further the control may be a current limitation control for limiting the current than usual based on the detection temperature of the temperature sensor (for example, control for decreasing an upper limit value of the current only a ratio according to the detection temperature, the current does not always decrease every time).
According to the invention, as the temperature sensor detects the temperature of the designated portion such as the above-mentioned, the temperature of the relay can be judged and monitored based on detection temperature of the temperature sensor. Based on the result of monitoring, the current correction control is carried out for correcting down the current flowing through the relay according to necessity, and the temperature of the relay is kept less than the allowable temperature.
Accordingly, it is positively and surely avoided that the relay deteriorates or is damaged by the overheat. Furthermore as the current being the source of the heat is limited, the relay is protected surely from the overheat without providing a large radiator or an enlarging capacity (size) of the relay. Therefore, the device can make sure of high reliability avoiding enlarging the device.
An electric power steering apparatus according to the invention has an auxiliary motor which is connected to a steering system of a vehicle and generates a steering-assistance torque, a bridge circuit which comprises plural switching elements and drives the auxiliary motor, and a relay which is provided at a current-carrying line of the auxiliary motor. The apparatus cuts the current-carrying line by making the relay an open state when an abnormal condition to cut the current-carrying line occurs. At a usual operation condition not generating the abnormal condition, the apparatus controls the current of the auxiliary motor to a target current value so that the steering-assistance torque becomes a value corresponding to the steering torque, by operating the switching elements according to the steering torque of the steering system while the current-carrying line with an conductible condition is kept by making the relay a close state. The apparatus has a temperature sensor for detecting a temperature at a designated portion where a temperature of the relay can be detected or calculated, and a current control means for carrying out a current correction control for correcting down the target current value according to necessity based on the detection temperature detected by the temperature sensor in order to keep the temperature of the relay less than an allowable temperature.
Here in xe2x80x9ccurrent-carrying line of the auxiliary motorxe2x80x9d, a current-carrying line (current path) between the bridge circuit and the power source and a current-carrying line (current path) between the bridge circuit and the auxiliary motor are included.
According to the invention, as the temperature sensor detects the temperature of the designated portion such as the above-mentioned, the temperature of the relay is judged and monitored based on the detection temperature of the temperature sensor. Based on the result of monitoring, the current correction control is carried out for correcting down the target current value of the auxiliary motor according to necessity. Accordingly, the temperature of the relay is kept less than the allowable temperature.
Because of that, although the steering-assistance torque is decreased as rate as the limitation and the steering operation becomes heavy as rate as the torque, it is positively and surely avoided that each relay deteriorates or is damaged by the overheat. Further as the current being the source of the heat is limited, each relay is protected surely from the overheat with out providing a large radiator and making the capacity (size) of the relay large. Accordingly, the apparatus can keep the high reliability avoiding enlarging the control unit of the electric power steering apparatus. Result of this, the high reliability electric power steering apparatus protected surely from the overheat of the relay can be realized even for the comparatively large vehicle such as a normal personal car in which the large current is necessary to flow for driving the auxiliary motor. Further the apparatus has practically a superior effect corresponding to needs of market that the unit is constructed with a suitable size for mounting the control unit of the apparatus onto the vehicle.
A preferred construction of the invention is that the temperature sensor measures a temperature of a specific part of the relay as the temperature of the designated portion.
According to the construction, the temperature of the specific part of the relay is measured directly, and the temperature is monitored correctly and easily to protect the overheat of the relay.
A preferred construction of the invention is that the temperature sensor measures a temperature of a board mounting the relay thereon as the temperature of the designated portion.
According to the construction, as the temperature sensor is provided by mounting on the above-mentioned board, an attachment of the temperature sensor becomes easy.
A preferred construction of the invention is that the board (board mounting a relay) is a board made by metal.
According to the construction, the heat generated at the relay radiates efficiently outside through the board. By that the overheat of the relay is hard to generate, and the operation frequency and degree of the above-mentioned current correction control becomes a little so as to obtain an effect easing the harmful effect of the current correction control (harmful effect that the steering operation becomes heavy as degree as correcting down the target current value). In this case, as the temperature of the board measured by the temperature sensor becomes more desirable on the point of judging the temperature of the relay (that is, a relation between temperature of the relay and temperature of the board become more accruable), and it becomes possible to judge and monitor accurately the temperature of the relay so as to show effectively and efficiently action and the effect of the current correction control.
Particularly when the relay is surface-mounted on the board, the heat generated at the rely efficiently transfers to the board, and the heat generated at the rely efficiently radiates outside through the board so that the above-mentioned action and effect become more clear.
A preferred construction of the invention is that the current control unit carries out the current correction control when the detection temperature detected by the temperature sensor rises over a predetermined value.
According to the construction, the judging process is simplified whether the current correction control is carried out or not.
A preferred construction of the invention is that the current correction control raises a degree for correcting down the target current value step by step or continuously according to increase of the detection temperature or below-mentioned temperature assessed value.
According to the construction, for example, when a degree of a high temperature state of the relay is comparatively low level as not so urgent, a flexible control is possible that a degree for limiting the current is controlled to be small and as a large steering-assistance torque as possible is kept. Especially in the case of increasing the correction level continuously, a fine current correction according to the degree of the high temperature state becomes possible, and it becomes finely possible that the overheat of the relay is surely avoided with as necessary minimum current correction as possible and on the other hand as large steering-assistance torque as possible is generated.
A preferred construction of the invention is that a temperature rise caused by Joule heat at the relay from the current flowing through the relay is calculated, the detection temperature detected by the temperature sensor is added to the temperature rise, a temperature assessed value of the relay is calculated, and when the temperature assessed value rises over a predetermined value, the current control means carries out the current correction control.
According to the construction, the temperature detected by the temperature sensor is a base temperature at assessment of temperature of the relay. Even if the temperature sensor does not measure the temperature of the relay itself and the temperature of a part correlating therewith (for example, even if the temperature is detected at a place separating some degree from the relay on the board) , the temperature assessed value of the relay is calculated correctly enough (in allowable error) so as to show action and effect of the current correction control with fully actual level. Therefore a degree of freedom of the arrangement of the temperature sensor increases, and it becomes easily possible to use the temperature sensor for another object (for example, the temperature monitoring of the switching element constructing the bridge circuit).